1 Technical Field
The present invention relates to rotating electric machines which include an armature and a field rotor (i.e., a rotor functioning as a field).
2 Description of Related Art
To achieve a small size, high performance, long service life and high reliability, permanent magnet motors, particularly IPM (Interior Permanent Magnet) motors with permanent magnets embedded in a rotor have been widely used. The IPM motors can greatly change, by changing the amplitude or phase of armature current, not only torque but also the amount of field magnetic flux passing through the armature. Therefore, IPM motors have advantages such that it is possible to control a counterelectromotive force and cause them to exert high torque during, for example, high-speed rotation.
Japanese Patent Application Publication No. JP2015526052A discloses a variable magnetomotive force rotating electric machine that is designed to obtain stable torque without employing expensive high-coercivity magnets and without performing a field-weakening control in a high-speed region. Specifically, the variable magnetomotive force rotating electric machine includes a rotor that has magnetic gap portions each being formed between a permanent magnet arranged in a d-axis magnetic path of one pole and an adjacent permanent magnet of a different polarity. With the magnetic gap portions, there are also formed, in the rotor, d-axis bypasses via which d-axis magnetic flux passes through an area other than the permanent magnets. Moreover, with the d-axis bypasses, the magnetic reluctance in a d-axis direction becomes lower than the magnetic reluctance in a q-axis direction.
However, in the prior art rotating electric machines including the variable magnetomotive force rotating electric machine disclosed in the above patent document, it is required for permanent magnets to be capable of withstanding demagnetization and be supplied with high magnetic flux. Therefore, it is needed to employ permanent magnets having high coercivity and high magnetic flux density. As a result, the manufacturing cost is increased.
Moreover, with the employment of permanent magnets having high magnetic flux density, when antiphase current for weakening the field is supplied to the armature coil (or stator coil) and thus the flux linkage of the armature coil is reduced, an amount of magnetic flux corresponding to the reduction in the flux linkage is leaked to other areas (e.g., to the d-axis bypasses disclosed in the above patent document). Consequently, there still exists magnetic flux passing through the armature core (or stator core). That is, though the voltage is lowered, the amount of magnetic flux passing through the armature core is not reduced. As a result, there still remains the problem that the iron loss is large during, for example, high-speed rotation.